1. Field of the Invention
The present invention relates to cement compositions, and more particularly to the use of hydraulic cement compositions for sealing or cementing subterranean zones penetrated by a borehole, such as cementing the annular space between an oil and/or gas well casing and the surrounding formation. In particular, the invention relates to an improved hydraulic cement slurry in which a stabilized and dispersed gas is generated at a controlled rate for cementing zones which contain gas or fluid under pressure, so that emission and flow of gas or fluid from the formation into the borehole or well annulus is suppressed or controlled by the counteractive pressure resulting from the inclusion of gas in the cement slurry prior to the time that the cement composition sets to a hardened state.
2. Description of the Prior Art
In the production of hydrocarbons from a subterranean formation, the subterranean formations are typically cemented or sealed by pumping an aqueous hydraulic cement slurry into the annulus between the pipe and the formation. In the oft practiced placement of cement in the annular space between the casing of an oil well and the surrounding subterranean formation, the cement slurry is commonly pumped into the casing and back up the annular space outside the casing. Occasionally, the cement is introduced directly to the annular space at the outer side of the casing. Where the cement has been pumped down the casing initially, any cement slurry which remains in the casing is displaced into the annulus by a suitable fluid or fluids.
On some occasions, the zones adjacent the cement containing annulus contain connate gas under substantial pressure. In these instances, an undesirable phenomenon referred to in the art as gas leakage is sometimes encountered in which the formation gas enters the annular space which surrounds the well casing after the primary cementing slurry has been placed. This gas can migrate to the surface, or other subterranean zones, through the annulus and the cement, forming a permanent flow channel or a highly permeable cement, and the leakage of such gas continues even after the cement slurry has taken a final set. Such gas leakage is detrimental to the long term integrity and sealing efficiency of the cement in the annulus, and the magnitude of such leakage is often enough to require an expensive remedial squeeze cementing job to be carried out to suppress or stop the gas leakage. Such gas leakage can cause high volume blow-outs shortly after the cement placement and before the cement has initially set.
Gas leakage occurs even though the initial hydrostatic pressure throughout the column of the cement slurry placed in the annulus far exceeds the pressure of gas in the formation from which the leaking gas originates. In explanation, it is theorized that two different well bore conditions can occur which will allow gas entry into the annulus. The first condition which is believed to be a prerequisite for annular fluid-gas migration is gellation of the cement slurry and subsequent development of static gel strength. This condition starts shortly after the cement slurry becomes static. The pressure required to move the cement is then directly related to the column length and the static gel strength. Thus as static gel strength increases, there is a loss of ability to transmit hydrostatic pressure.
The second condition which contributes directly to the loss of pressure in the cement column (and across the pressurized gas zone) is the loss of fluid and volume reduction within the cement column. This condition is believed to be due to the leak-off of water in the cement into the formations and from cement volume reduction due to chemical hydration.
Volume reductions occurring after static gel strength starts to develop results in a loss of pressure in the cement column. As the pressure in the cement column drops below the gas pressure, gas will enter the annulus. If at this time the static gel strength is still below the gas percolation value, a gas leakage condition is created.
Interestingly, the gelled or partially set cement, although it is incapable of maintaining or transmitting full hydrostatic pressure, still is not sufficiently rigid or set to prevent the entry of gas into the annulus and the upward percolation of the gas. According to the most popular theories, an absolute volume reduction occurring after the cement column can no longer transmit full pressure reduces the pore pressure of the still semi-plastic slurry. When the pore pressure falls below the formation gas pressure, formation gas leaks into the well bore and if the cement is not gelled enough to prevent percolation, gel leakage channels are formed. Two principal mechanisms which act to decrease the pore pressure are the hydration reaction of cement and the loss of filtrate to the adjacent permeable formation.
Gas leakage problems have been noticed following casing cementing operations on surface conductors and intermediate, production, and liner jobs. Gas returns to the surface have often been noticed within one to seven hours after placement of the cement. Many times, however, the gas flow does not return to the surface, but flows into low pressure zones causing interzonal gas communication.
In U.S. Pat. Nos. 4,304,298, and 4,340,427, which are assigned to the assignee of the present application and are herein incorporated by reference, in an effort to prevent gas leakage, the use of a cement slurry containing a stabilized, dispersed gas is described. Enough gas is present in the cement slurry of this invention to prevent gas under pressure from passing into or around the cement prior to the time the cement has set or gelled sufficiently to prevent percolation. The entrained gas, by virtue of its compressibility, reduces the magnitude of the pressure decrease resulting from the slurry volume reduction.
In U.S. Pat. No. 4,367,093, which is also assigned to the assignee of the present application and is herein incorporated by reference, a composition and method for cementing adjacent to a subterranean formation is disclosed. The cementing composition of the invention comprises a hydraulic cement, aluminum powder to generate hydrogen gas, and an inhibitor which retards the generation of hydrogen gas produced by the aluminum powder.